Cryogenic Cooling Method Using a Gas-Solid Diphasic Flow of CO2

ABSTRACT

The invention relates to a method implementing liquid CO 2  as a cryogenic fluid for transferring negative calories to products, said method being of a so-called indirect injection type wherein the liquid CO 2  is sent into a heat exchanger system where same evaporates, the method being characterized in that, prior to reaching the exchanger system, the liquid CO 2  undergoes an expansion operation, at a pressure selected to obtain a gas/solid mixture at the output of the expansion operation.

The present invention relates to the field of the processes using CO₂ as cryogenic fluid in processes for the cooling, deep-freezing and case-hardening of products, in particular foodstuffs, but also as source of cold in refrigerated trucks which transport fresh and/or deep-frozen (thus heat-sensitive) products.

In such processes and applications, the CO₂ is most often intended to be used in direct injection, with temperatures for regulating the products to be cooled which typically vary between 0 and −20° C., in the case of refrigerated transport, and between −40° C. and −70° C., in cooling cells and other cooling tunnels.

While the use of CO₂ in direct injection exhibits unquestionable advantages, in particular the absence of thermal barrier and consequently the guarantee of maximum thermal efficiency, on the other hand it exhibits disadvantages, among which may be mentioned:

-   -   the question of safety, it requires the installation of devices         making it possible to prevent the risk of asphyxia (alarm         systems, extraction systems, CO₂ sensors), with the costs and         constraints which this involves;     -   from the thermodynamic viewpoint: the heat of the extraction         gases, in particular those at −40° C./−70° C., are difficult to         recover in value as, after they have come into direct contact         with the products to be cooled, they become contaminated by the         presence of traces of moisture, of particles of products, and         the like.

However, there are also numerous applications where CO₂ is used in indirect injection in an open loop, in particular in applications for refrigerated transportation but also in deep-freezing tunnels; where a heat exchanger is fed with liquid CO₂ which, when evaporating in this exchanger, extracts the heat from the medium to be cooled and thus produces the desired cold (the transfer of the cold to the products involves an exchange with the internal air of the tunnel or the truck via ventilation means associated with each exchanger). Use is thus made here of a Liquid/Vapor phase change which, from the viewpoint of the thermodynamic properties of the CO₂, is “restricted” to a theoretical pressure of 5.18 bar corresponding to the pressure of the triple point of this fluid. In other words, the temperature at which the phase change takes place is found to be limited and, in all cases, it is strictly greater than −56.6° C. The demonstration is thus made of the fact that the use of CO₂ in indirect injection does not make it possible to achieve very low temperature levels, in contrast to what is made possible by liquid nitrogen, for example.

The present invention hopes to provide novel conditions for the use of CO₂ as source of cold in such indirect injection applications.

As will be seen in more detail below, the invention provides for the installation of a gas-solid two-phase flow.

The invention relates to a process employing liquid CO₂ as cryogenic fluid, making it possible to transfer cold to products, process of the “indirect injection” type where liquid CO₂ is conveyed into a heat exchanger system where it evaporates, the transfer of cold to the products involving an exchange between the air surrounding the products and the cold walls of the heat exchanger, promoted by the involvement of ventilation means associated with the heat exchanger system, the process being characterized in that, before reaching the exchanger system, the liquid CO₂ has been subjected to an operation for reducing in pressure to a pressure chosen in order to obtain, at the outlet of the pressure-reducing operation, a solid/gas mixture.

According to a preferred embodiment of the invention, before reaching the pressure-reducing operation, the liquid CO₂ has been heat-exchanged with the cold gases obtained at the outlet of the heat exchanger system (resulting from the melting carried out in the heat exchanger system).

This heat exchange between the liquid CO₂ and the cold gases obtained at the outlet of the heat exchanger system is, for example, carried out in a plate exchanger.

The following will thus have been understood, on reading the above:

-   -   there is sent, into the exchanger of this indirect injection         process, not, as according to the prior art, liquid CO₂ but a         fluid resulting from a reduction in pressure, in which there is         a part of solid (this is a gas/solid two-phase liquid);     -   and the advantageous embodiment of the invention explained         above, where, before being sent to the pressure-reducing valve,         the liquid exchanges with the gas phase extracted from the heat         exchanger system (which is a way of subcooling this liquid),         offers a higher thermal efficiency since the solid fraction in         the liquid which has been subcooled and then reduced in pressure         is then greater.

Other characteristics and advantages of the present invention will thus become more clearly apparent in the following description, given by way of illustration but without implied limitation, made in connection with the appended drawings, for which:

FIG. 1 is a partial diagrammatic representation of an embodiment of the invention;

FIG. 2 presents enthalpy difference curves which make it possible to visualize the difference in enthalpy between points 2 and 3 of FIG. 1, including the latent and sensible heats, for two pressure levels, 5.18 bar (triple point pressure) and 1 bar;

FIG. 3 is a partial diagrammatic representation of an advantageous embodiment of the invention employing subcooling of the liquid CO₂ before it arrives in the pressure-reducing valve.

FIG. 1 makes it possible to visualize, in a simple and clear way, the progress of the liquid CO₂ in a process in accordance with the invention. If necessary, but without in any way being obligatory, reference may be made, to better follow that which follows, to a Mollier diagram, a diagram well known to a person skilled in the art, but which the applicant company has chosen not to display here for reasons of readability.

As may be read in FIG. 1, the liquid CO₂ (point 1) withdrawn from the storage tank, for example under standard conditions of 20 bar/−20° C. type (or also 45° C./8 bar type, depending on the country concerned), is reduced in pressure to a pressure below that of the triple point, for example below 5.18 bar (point 2), before reaching the exchanger system.

The exchanger system is employed in an “indirect injection” process: for example in an operation for cooling, deep-freezing or case-hardening products, in particular foodstuffs (the exchanger system is then, for example, present inside a cryogenic cell or tunnel), or in a refrigerated truck transporting perishable heat-sensitive products.

There is thus obtained, at point 2, a gas/solid two-phase mixture, the solid fraction of which varies as a function of the pressure at point 2. By way of illustration, it is typically 52% at 5.18 bar/−56.6° C. and 47% at 1 bar/−80° C.

This two-phase mixture is then circulated inside the exchanger system, where the mixture gives up its latent heat of fusion in addition to a portion of its sensible heat. The design of the exchanger and in particular its exchange surface, and also the CO₂ flow rate, will define the refrigerating capacity delivered and also the outlet temperature of the gas at point 3.

FIG. 2 exhibits enthalpy difference curves, making it possible to visualize the enthalpy difference between the points 2 and 3 of FIG. 1, including the latent and sensible heats, for two pressure levels after reducing the liquid CO₂ in pressure, 5.18 bar (i.e., the triple point pressure) and 1 bar.

This FIG. 2 clearly shows the available energy (expressed as enthalpy variation) present in one kilogram of CO₂ when the latter is reduced in pressure from 20 bar to 5.18 bar, representing the limit of the liquid/vapor phase change (bottom curve in the figure), or else from 20 bar to 1 bar (top curve in the figure), making it possible to obtain, in accordance with the invention, a solid/gas two-phase mixture. It is noted that, in both cases, the enthalpy variation increases in proportion as the outlet temperature of the gas also increases, and the fact that this enthalpy variation increases in proportion as the pressure after reducing in pressure decreases. Hence the indisputable energy advantage of what is provided by the present invention by the use of a solid/gas fluid instead of a liquid/gas fluid as according to the prior art.

Nevertheless, it should be mentioned that, for some food cryogenic applications, for example for certain products in deep-freezing applications in tunnels, the cryogenic temperature effect is keenly desired. Thus, in such applications, gases at such a high temperature can be obtained with difficulty at the exchanger outlet since the temperature of the air surrounding the products which is desired in such processes typically has to reach −60° C. to −80° C.

For such requirements, just as for other applications, it will then be very particularly advantageous to employ the advantageous embodiment of the invention which is illustrated in FIG. 3 below.

This advantageous form is targeted at being able to recover in value as much as possible of the heat still present in the gases extracted at the outlet of the exchanger system.

Let us examine the embodiment of FIG. 3.

In this figure, the presence of an additional means is noted; it is a means which makes it possible to carry out exchange of heat, to be specific a subcooler, for example composed, as is the case here, of a plate exchanger, the operation of which means will be explained here:

-   -   the liquid CO₂ (point 1) withdrawn from the storage tank, for         example under standard conditions already mentioned above in the         context of FIG. 1, passes, before reaching the pressure-reducing         valve, through a plate exchanger where it exchanges heat with         the gases resulting from the exchanger system (point 4),         exchange system present in the tunnel, or the truck, and the         like;     -   it is thus seen that the liquid CO₂ coming from the storage tank         (point 1) and the gases extracted from the heat exchanger system         (point 4) circulate countercurrentwise in the plate exchanger,         which makes possible the subcooling of the stream of liquid CO₂         before the latter reaches the pressure-reducing station (point         2);     -   between points 1 and 2, the liquid thus remains at a         substantially constant pressure but is subjected to cooling;     -   at the outlet of the pressure-reducing station (point 3), the         solid/gas mixture obtained is directed towards the heat         exchanger system;     -   the gases extracted from the heat exchanger system (point 4),         once they have passed through the subcooler, are discharged         (point 5);     -   the production of cold thus takes place in the exchanger system         between points 3 (after reducing in pressure) and 4 (exchanger         outlet).

As indicated above, the temperature at this point 4 will be dictated by the technical constraints of the application which uses the cold, which makes it possible to result in a higher or lower level.

Two examples of conditions and compositions of phases at the various points 1, 2, 3, 4 and 5 of FIG. 3 are described in detail below.

FIRST EXAMPLE

T P h Point (° C.) (bar) (kJ/kg) Thermodynamic state 1 −20 20 40.8 Liquid/vapor equilibrium 2 −34 20 13.1 Subcooled liquid 3 −80 1 13.1 Gas/solid mixture 4 −60 1 323.5 Superheated gas 5 −25 1 351.2 Superheated gas

SECOND EXAMPLE

T P h Point (° C.) (bar) (kJ/kg) Thermodynamic state 1 −20 20 40.8 Liquid/vapor equilibrium 2 −52 20 −22.4 Subcooled liquid 3 −80 1 −22.4 Gas/solid mixture 4 −80 1 288.0 Gas/solid mixture 5 −25 1 351.2 Superheated gas

This second example illustrates a case where, if the application which uses the cold requires a temperature of the medium to be cooled which is as cold as possible, it is possible to envisage making partial use of the heat of fusion in the exchanger system (between points 3 and 4), the complete melting of the mixture and the superheating thereof then taking place in the subcooler with recovery of the heat.

In other words, by varying the exchange surface of the exchanger, it is possible to carry out partial melting in the exchanger, a solid/gas mixture thus exiting at point 4; there then takes place, in the exchanger, a change of state of melting, which takes place at the constant temperature for a pure fluid, such as CO₂ (in the form illustrated here, it is not the temperature which changes but the fraction by weight of the solid, which decreases as it goes along in order to be converted into vapor).

As will have been understood on reading all the explanations given above, the process according to the invention in its form of FIG. 3 makes it possible:

-   -   to increase the refrigerating capacity of the exchanger of the         indirect injection system since the subcooling of the liquid CO₂         makes it possible to gain up to 12% of available energy;     -   to improve the heat exchange as a subcooled fluid, once reduced         in pressure, gives rise to a greater solid fraction, which is         beneficial for the transfer coefficient.

If the application requires a temperature of the medium which is as cold as possible, it is possible to envisage making partial use of the heat of fusion in the process (between points 3 and 4), the complete melting of the mixture and its superheating then taking place in the subcooler with recovery of the heat. 

1-6. (canceled)
 7. A process employing liquid CO₂ as cryogenic fluid, for transferring cold to products, process of the “indirect injection” type where liquid CO₂ is conveyed into a heat exchanger system where it evaporates, the transfer of cold to the products involving an exchange between the air surrounding the products and the cold walls of the heat exchanger, promoted by the involvement of ventilation means associated with the heat exchanger system, the process being characterized in that, before reaching the exchanger system, the liquid CO₂ has been subjected to an operation for reducing in pressure to a pressure chosen in order to obtain, at the outlet of the pressure-reducing operation, a solid/gas mixture.
 8. The process of claim 7, wherein before reaching the pressure-reducing operation, the liquid CO₂ exchanges heat with the cold gases obtained at the outlet of the heat exchanger system, in a means which makes possible such a heat exchange.
 9. The process of claim 8, wherein the exchange surface of the exchanger system is given dimensions so as to carry out, in the exchanger, only a partial melting of the entering gas/solid mixture, the complete melting of the mixture then taking place in said means which makes possible heat exchange.
 10. The process of claim 8, wherein said means which makes possible heat exchange is a plate exchanger.
 11. A plant for the transfer of cold to products using liquid CO₂, the plant employing a process of the “indirect injection” type and comprising: a heat exchanger system capable of passing liquid CO₂ in transit therein; and ventilation means associated with the heat exchanger system, capable of bringing the air surrounding the products into contact with the cold walls of a heat exchanger system, the plant being characterized in that it comprises a pressure-reducing system, positioned upstream of the exchanger system, thus capable of reducing the liquid CO₂ in pressure, before it arrives in the exchanger system, to a pressure chosen in order to obtain a solid/gas mixture at the outlet of the pressure, reducing operation.
 12. The plant of claim 11, wherein it additionally comprises a subcooling system, for example a plate exchanger, positioned in the plant according to the following arrangement: the subcooling system is positioned between the source of liquid CO₂ and the pressure-reducing system, in order to make it possible for the liquid CO₂ to be able to pass in transit by a first pathway of this subcooling system, before reaching the pressure-reducing system; said arrangement is furthermore such that it makes it possible for the cold gases extracted from the heat exchanger system to pass in transit by a second pathway of the subcooling system. 